The present invention relates to a hybrid propulsion system, in which the driving wheels of a self-propelled vehicle can be driven by both an electric driving motor supplied by a rechargeable battery and an internal-combustion engine.
A typical, although not sole example of self-propelled vehicle which is particularly suited to the use of this propulsion system is constituted by the motor-sweepers, ie. the machines that are used to clean large surfaces both outdoors (where most often it is the internal-combustion engine that is preferably used) and indoors (where use is made solely of the electric driving motor).
Patent application EP-A-0 887 222 discloses a modern self-propelled vehicle whose hybrid propulsion system comprises:
a rechargeable battery,
a so-called driving motor, which is constituted by a conventional electric direct-current motor, to drive both the driving wheels and the brushes,
an internal-combustion engine,
a permanent-magnet motor rotatably connected with the internal-combustion motor;
control means that are also adapted to detect the state of charge of the battery.
Under battery-supply operating conditions, the internal-combustion motor is turned off and the movement of the vehicle is ensured by the afore cited electric driving motor, which is supplied from the battery itself.
The control means, when after a prolonged period of operation from battery they detect that the charge of the battery is decayed below a pre-determined level, divert the flow of current therefrom towards the permanent-magnet motor which, owing to its being connected rotatably with the internal-combustion engine, is in this way caused to act as a starting motor for the latter.
Upon being so started, it is then the internal-combustion engine that drives rotatably the permanent-magnet motor, so that the latter is able to act as a generator of an electric current that supplies the electric driving motor so as to ensure the operation of the vehicle. A possible surplus of current generated by the permanent-magnet motor in excess of the actual current input to the driving motor, is used to recharge the battery.
As soon as the battery is again charged to its rated value, the above cited control means cause the internal-combustion engine to automatically turn off and the vehicle starts again to operate regularly from the battery as described above.
When the vehicle, during the operation of the internal-combustion engine, finds itself working under particularly difficult or hard conditions, such as for instance when it has to climb over a slope or ramp, the control means make sure that the electric driving motor is at the same time energized by the current generated by the permanent-magnet motor and the current supplied by the battery. Thanks to this buffer or boost function performed by the battery in these circumstances, a correspondinyly increased power becomes in this way available at the driving wheels so as to therefore enable the vehicle to get over the obstacle.
Although offering a number of undisputable advantages from both a construction and an operation point of view, this prior-art system of hybrid propulsion has a first drawback in the fact that the permanent-magnet motor operates to perform such duties as to start the internal-combustion engine, recharge the battery and supply a part (which may be more or less important, depending on the charge level of the battery) of the energy needed to sustain the operation of the vehicle. In other words, the permanent-magnet motor, which is a particularly useful, but also quite expensive component part, is actually able to operate at the top of its potential only when the charge of the battery is at a very low level, and this clearly puts a penalty on the overall efficiency of the system.
Furthermore, in the above described prior-art system the battery is not only subject to repeated discharge and charge phases, but also runs the risk of fully losing its charge, ie. getting fully exhausted, if, after the fuel has run fully out, the driver, the operator in charge of the vehicle fails to immediately refuel, since the battery supplies also the permanent-magnet motor that forcedly drives the internal-combustion engine into rotation. Apart from the impossibility for the work to be completed, this brings about a twofold drawback, ie. the need for the internal-combustion engine to be at this point started solely by hand, after refuelling, and the need for the battery to be recharged by means of a separate battery-charger.
It is a purpose of the present invention to provide a hybrid propulsion system that not only has a high efficiency, but also does away with the above cited drawbacks, as well as a self-propelled vehicle using such a hybrid propulsion system.
A propulsion system and a self-propelled vehicle with the characteristics as recited in the appended claims enable such an aim to be reached.